On Sun, 6 Sep 2009, Jim Brown wrote:
> Paul, W9AC wrote:
>
>> Looks like this series of Corcom filter may be worth trying on a washer with
>> a VFD motor.
>
>> http://www.cor.com/pdf/FC.pdf
>
>> I just ordered one for my GE Harmony Profile-series washer. It's now out of
>> warranty, so I can try some more invasive measures. To date, nothing has
>> made a dent in reducing RFI/EMI from its direct-drive motor circuitry. I've
>> got a feeling that a total cure will involve attacking the noise at multiple
>> points along the VFD circuit.
>
> Remember that a line filter only kills trash coming out the power cord. If the
> unit is not well shielded (I believe Chris said some he tested are not), you
> WILL need to deal with the VFD circuit.
Some of the washers I inspected had entire panels of plastic with a metal
frame. In my case the chassis is 100% steel with plastic facing, the
"computer" is more or less boxed, and the VFD is 8 inches from the motor
with a 12in power lead, not twisted but multiple 12ga insulated conductors
in the same outer jacket.
I saw a few with 18 to 24 inch leads, where the conductors were totally
seperate and running via different paths.
> One very good start is to study the current path that drives that motor and
> use twisted pair cable for as much of it as possible. Another is to make sure
> that "ground" or the chassis is not part of that path, so that the magnetic
> field produced by that current is confined to the small region around that
> twisted pair.
>
> In other words, you do NOT want the chassis or "ground" to be part of the
> current path, either at the fundamental frequency or at the high harmonics
> (RF) that are causing us grief. We want both conductors that carry current
> between the controller and the motor to be "above ground." This seems
> counterintuitive -- we've been taught to "ground" everything as a cure for RFI
> -- but tying one side of the motor winding to ground, or adding capacitive
> coupling to "ground" causes current to flow in a big loop rather than a small
> one, so it produces a bigger field and does more radiation. When the current
> on the "hot" conductor is precisely equal to the current on the "return"
> conductor and they are very close to each other, both magnetic fields and
> radiation are minimized.
I strongly suggest _NOT_ even thinking about bypassing the motor leads to
*anything*, unless you know the exact operating params of the VFD and
really, really know what you are doing. Variable frequency PWM square
wave...
I have never done it myself, but I have seen what happens when a several
KW peak motor controller of a similar type is bypassed with too large a
cap, cap go boom... If you know the max frequency of operation, the
distribution and total amount of harmonic energy and peak voltages you
could select the correct values but this is not in the manual.
Any filters, bypassing, whatever needs to be on the input side of the
motor controller, not the output. The exception would be replacing an
over long split run (read antenna) to the motor with twisted pair of the
proper length.
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